The thermodynamic study of (4a alpha, 8a beta, 9a beta, 10a alpha)-tetradecahrdro-10-methyl-acridine (I),(4a alpha, 8a beta, 9a beta, 10a alpha)-10-ethyl-tetradecahydroacridine(II), (4a alpha, 8a alpha, 9a beta, 10a alpha)-tetradecahydro-10-methyl-acridine(III) and (4a alpha 8a alpha, 9a beta, 10a alpha)-10-ethyl-tetradecahydroacridine(IV) in condensed stale has been performed by combustion calorimetry of small amounts of substance and by sublimation calorimetry using a CRMT and Tian-Calvet calorimeter, respectively. Enthalpies of combustion and sublimation art experimentally determined. The formation enthalpies in solid and gaseous stales were calculated. These energetic values were used for the determination of isomerisation energies between different stereoisomers and also for the atomization energies of thr studied molecules. The saturated vapour pressures of these four compounds have been estimated.